1. Field of the Invention
The present invention relates to developing apparatus for use in electrophotographic copying machines or electrostatic copying machines. More particularly, the invention relates to a developing apparatus adapted to hold the surface of its developing roller in contact with the surface of a photosensitive drum under a controlled constant pressure.
2. Description of the Prior Art
With electrophotographic copying machines or electrostatic copying machines, the surface of an image bearing photosensitive drum is uniformly charged first and then exposed to an optical image of an original to form a corresponding electrostatic latent image. Subsequently, the latent image is developed to a visible image with toner supplied to the image bearing drum surface from a developing apparatus. The toner image obtained is thereafter transferred and fixed to paper or like material.
It is very important to restrict the minute gap between the photosensitive drum (electrostatic latent image bearing member) and the developing sleeve in the developing apparatus for use in such electrophotographic copying machines, especially those of the impression type, touchdown type or like contact development type wherein nonmagnetic toner is used as a single-component developer, or those of the noncontact jumping development type.
To obtain satisfactory toner images with the contact developing apparatus wherein the single-component developer consisting only of a toner is brought into contact with the photosensitive drum, with the surface-to-surface distance between the drum and the developing roller set to not greater than the thickness of the toner layer on the roller, it is critical to hold the developing roller in contact with the drum, a toner layer restricting plate or toner scraping-supplying roller under a uniform pressure with good stability. First, the layer of toner supported on the developing roller surface and to be supplied to the drum must have a uniform thickness and be charged to a uniform potential. The pressure to be exerted by the blade for restricting the thickness of the toner layer must therefore be stably controlled to a uniform level over the entire width. Further to maintain constant developing conditions, it is necessary to hold the developing roller in contact with the photosensitive drum under a constant pressure with a contant nip width. To ensure a constant toner scraping-suppying condition, the toner scraping-supplying roller needs to be held in contact with the developing roller under a uniform pressure.
Thus, in the case where single-component or two-component developers are used, it is required to control the gap between the developing roller and the drum to a constant value or to maintain the roller in contact with the drum under a uniform and constant controlled pressure, whereas substantial difficulties have been encountered in controlling the position of the roller relative to the drum as desired.
The gap between the photosensitive drum and the developing roller is conventionally controlled, for example, by the following means. To control the gap to a value exceeding 0.5 mm, the position of the drum and the position of the roller are adjusted by an integral contact restricting member for supporting the bearings members for these drum and roller. The gap is restricted to about 0.5 to about 0.3 mm by a fixed spacer as disclosed in Examined Japanese Patent Application No. SHO 62-45553. U.S. Pat. No. 3,987,756 discloses a roller mounted on the same shaft as the developing roller and having a larger diameater than the developing roller in corresponding relation to the desired gap for use in the magnetic brush developing system employing a two-component developer to provide a controlled gag (400 to 2,000 .mu.m), or for use in the jumping developing system to afford a minute controlled gap (100 to 250 .mu.m).
Nevertheless, the control by these means involves limitations due to the accumulation of machining dimensional errors and encouters extreme difficulties in stably restricting the gap to not greater than 0.1 mm. Moreover, the torsion or the like of the shafts concerned relative to each other also results in an uneven gap. The conventional means are therefore in no way satisfactorily usable for giving a uniformly controlled gap.
Especially for use in the contact developing process wherein a thin layer of toner on the developing roller is brought into contact with the photosensitive drum, none of the conventional developing apparatus are adapted to readily afford a uniform contact pressure between the developing roller and the drum, the toner restricting plate or the scraping roller.